The QT interval represents ventricular depolarization and repolarization. Because it varies with heart rate, the corrected QT (QTc) standardizes it. Prolonged QTc increases risk of torsade de pointes arrhythmia.
Formulas
- Bazett: QTc = QT / √RR (where RR = 60/HR)
- Fridericia: QTc = QT / ∛RR
Both formulas express QTc in milliseconds.
Interpretation
- ≤440 ms: Normal
- 440–500 ms: Borderline prolonged
- >500 ms: Significantly prolonged — increased TdP risk
Bazett vs Fridericia
Bazett is the most commonly reported but tends to overcorrect at high heart rates and undercorrect at low ones. Fridericia is more accurate at extreme heart rates and is often preferred for drug studies and tachycardic patients.
Common QT-Prolonging Drugs
- Antiarrhythmics: Amiodarone, sotalol, dronedarone
- Antibiotics: Macrolides, fluoroquinolones
- Antipsychotics: Haloperidol, ziprasidone, quetiapine
- Antiemetics: Ondansetron, droperidol
- Methadone
Reference: CredibleMeds.org for full list.